Foreign plants such as annual bluegrass are establishing themselves on Antarctica, whose status as the coldest and driest continent had long made it one of the most pristine environments on Earth.

But a boom in tourism and research activities to the Antarctic Peninsula may be threatening the continent's unique ecosystems, scientists say. (See a high-res Antarctica map.)

For the study, ecologist Steven Chown at the University of Stellenbosch in South Africa and colleagues vacuumed the clothes, footwear, bags, and gear of approximately 2 percent of people who visited during the Antarctic summer from late 2007 to early 2008. That amounted to 853 scientists, tourists, and accompanying support workers and ships' crew members. (Read more about the gear required for Antarctic travel.)

"Endless hours were spent vacuum-cleaning clothes and gear. ... If one is doing so on a ship underway on a rough ocean, it can take a strong stomach," Chown recalled.

The results revealed more than 2,600 seeds and other detachable plant structures, or propagules, had hitched a ride to Antarctica on these visitors.

On average, tourists each carried two to three seeds, while scientists each carried six. However, the annual number of tourists now far outnumbers that of scientists—about 33,000 tourists to about 7,000 scientists in the 2007-2008 Antarctic summer. As a result, tourists and scientists likely pose similar risks overall to Antarctica, Chown said.

Based on the nature of these foreign species and the present climate of Antarctica, the areas at highest risk are the Antarctic Peninsula coast and surrounding islands, the study said.

According to climate projections for 2100 from the UN's Intergovernmental Panel on Climate Change (IPCC), invaders may also take root in the coastal, ice-free areas to the west of the Amery Ice Shelf and, to a lesser extent, in the Ross Sea region.